The present invention relates to a method of making an image sensor and the sensor so formed.
An image sensor is a device that transforms an optical image into an electric signal. Types of image sensors include a complementary metal-oxide-silicon (CMOS) image sensor and a charge coupled device (CCD) image sensor, as examples. In either case, associated with a pixel of an image sensor are photodiodes for receiving light and transistors for controlling image signals input from the photodiodes. In the photodiodes, electron-hole pairs are generated according to wavelengths and strengths of red, green, and blue lights irradiated through color filters having red, green, and blue colors, respectively. An output signal is changed according to an amount of the generated electrons. In this manner, an image may be sensed.
The color filters are formed by carrying out photolithography processes three times on a bottom planarization layer of a semiconductor substrate, one time for each color filter. This forms a color filter array. A photolithography process includes very complicated processes, such as a coating process for coating a photoresist onto a wafer, a soft bake process for removing solvents contained in the photoresist after coating the photoresist, an exposure process for aligning a photo mask on a wafer coated with the photoresist and irradiating the photoresist with lights through the photomask, a post-bake process for hardening the photoresist, and a developing process for developing the photoresist according to a photoresist pattern. Accordingly, a process of forming a color filter array in an image sensor may be complicated.
Since a color filter array having three colors is formed by performing a photolithography process for each color filter, it is difficult to control equally a thickness of color filters having different colors. Therefore, thicknesses of the color filters tend to differ, causing their heights to be significantly different, so their upper surfaces are not at substantially the same height. If micro lenses are directly formed on the color filters having different heights, it is difficult for micro lenses to have identical forms. This results in micro lenses having different focal distances, which decreases light-transmission uniformity among the pixels. In order to prevent this, before forming the micro lenses, an upper planarization layer having a flat upper surface is formed on the color filters. Adding the upper planarization layer increases the distance between the micro lenses and photodiodes, causing the amount of light reaching the photodiodes to be decreased. As a result, photosensitivity is reduced.